AN AB-INITIO STUDY OF THE SINGLET POTENTIAL-ENERGY SURFACE FOR THE REACTION OF NO3 WITH HO2

The geometries of transition states and intermediates on the singlet p otential-energy surfaces for the reactions NO, + HO2 --> OH + NO2 + O- 2 (1) and NO3 + HO2 --> HNO3 + O-2 (2) have been optimised in MP2 calc ulations with a 6-31G basis set. Energetics have been derived from si ngle point MP4/6-311G* calculations at the MP2/6-31G* geometries. Rea ction (1) proceeds without an overall activation barrier other than th e reaction endothermicity via three O2NO ... OOH rotational isomers. R eaction (2), which is exothermic, has no overall activation energy and involves hydrogen atom transfer in a NO3 ... HO2 complex which then y ields products via a HNO3 ... O-2 adduct. No evidence was found for a cyclic transition state.